LNT-SCR packaging

ABSTRACT

A power generation system comprises a diesel engine and a fuel reformer configured to receive the engine exhaust. Two or more separate LNT bricks are configured in a parallel valveless arrangement wherein each simultaneously receives a separate portion of the exhaust leaving the fuel reformer. The LNTs are each adapted and configured to simultaneously store NO x  when the exhaust from the fuel reformer is lean and to simultaneously reduce stored NO x  and regenerate when the exhaust from the fuel reformer contains reformate. This parallel multi-brick arrangement reduces the effective length to width ratio of the LNTs as a group without the packaging difficulties associated with a single LNT having an equivalently reduced length to width ratio. Axial temperature gradients that develop in the LNTs during desulfation are thereby mitigated.

PRIORITY

This application is a continuation-in-part of U.S. application Ser. No.11/223,589, filed Sep. 10, 2005.

FIELD OF THE INVENTION

The present invention relates to pollution control devices for dieselengines.

BACKGROUND

NO_(x) and particulate matter (soot) emissions from diesel engines arean environmental problem. Several countries, including the UnitedStates, have long had regulations pending that will limit NO_(x) andparticulate matter emissions from trucks and other diesel-poweredvehicles. Manufacturers and researchers have put considerable efforttoward meeting those regulations. Diesel particulate filters (DPFs) havebeen proposed for controlling particulate matter emissions. A number ofdifferent solutions have been proposed for controlling NO_(x) emissions.

In gasoline-powered vehicles that use stoichiometric fuel-air mixtures,NO_(x) emissions can be controlled using three-way catalysts. Indiesel-powered vehicles, which use compression ignition, the exhaust isgenerally too oxygen-rich for three-way catalysts to be effective.

One set of approaches for controlling NO_(x) emissions fromdiesel-powered vehicles involves limiting the creation of pollutants.Techniques such as exhaust gas recirculation and partially homogenizingfuel-air mixtures are helpful in reducing NO_(x) emissions, but thesetechniques alone are not sufficient. Another set of approaches involvesremoving NO_(x) from the vehicle exhaust. These approaches include theuse of lean-burn NO_(x) catalysts, selective catalytic reduction (SCR),and lean NO_(x) traps (LNTs).

Lean-burn NO_(x) catalysts promote the reduction of NO_(x) underoxygen-rich conditions. Reduction of NO_(x) in an oxidizing atmosphereis difficult. It has proven challenging to find a lean-burn NO_(x)catalyst that has the required activity, durability, and operatingtemperature range. Lean-burn NO_(x) catalysts also tend to behydrothermally unstable. A noticeable loss of activity occurs afterrelatively little use. Lean-burn NO_(x) catalysts typically employ azeolite wash coat, which is thought to provide a reducingmicroenvironment. The introduction of a reductant, such as diesel fuel,into the exhaust is generally required and introduces a fuel economypenalty of 3% or more. Currently, peak NO_(x) conversion efficienciesfor lean-burn NO_(x) catalysts are unacceptably low.

SCR generally refers to selective catalytic reduction of NO_(x) byammonia. The reaction takes place even in an oxidizing environment. TheNO_(x) can be temporarily stored in an adsorbent or ammonia can be fedcontinuously into the exhaust. SCR can achieve high levels of NO_(x)reduction, but there is a disadvantage in the lack of infrastructure fordistributing ammonia or a suitable precursor. Another concern relates tothe possible release of ammonia into the environment.

To clarify the state of a sometimes ambiguous nomenclature, in theexhaust aftertreatment art, the terms “SCR catalyst” and “lean NO_(x)catalyst” are occasionally used interchangeably. Where the term “SCR” isused to refer just to ammonia-SCR, as it often is, SCR is a special caseof lean NO_(x) catalysis. Commonly, when both types of catalysts arediscussed in one reference, SCR is used with reference to ammonia-SCRand lean NO_(x) catalysis is used with reference to SCR with reductantsother than ammonia, such as SCR with hydrocarbons.

LNTs are devices that adsorb NO_(x) under lean exhaust conditions andreduce and release the adsorbed NO_(x) under rich exhaust conditions. ALNT generally includes a NO_(x) adsorbent and a catalyst. The adsorbentis typically an alkaline earth compound, such as BaCO₃ and the catalystis typically a combination of precious metals, such as Pt and Rh. Inlean exhaust, the catalyst speeds oxidizing reactions that lead toNO_(x) adsorption. In a reducing environment, the catalyst activatesreactions by which adsorbed NO_(x) is reduced and desorbed. In a typicaloperating protocol, a reducing environment will be created within theexhaust from time-to-time to remove accumulated NO_(x) and therebyregenerate (denitrate) the LNT.

Creating a reducing environment for LNT regeneration involveseliminating most of the oxygen from the exhaust and providing a reducingagent. Except when the engine can be run stoichiometric or rich, aportion of the reductant reacts within the exhaust to consume oxygen.The amount of oxygen to be removed by reaction with reductant can bereduced in various ways. If the engine is equipped with an intake airthrottle, the throttle can be used. However, at least in the case of adiesel engine, it is generally necessary to eliminate some of the oxygenin the exhaust by combustion or reforming reactions with reductant thatis injected into the exhaust.

The reactions between reductant and oxygen can take place in the LNT,but it is generally preferred that the reactions occur in a catalystupstream from the LNT, whereby the heat of reaction does not cause largetemperature increases within the LNT at every regeneration.

Reductant can be injected into the exhaust by the engine fuel injectorsor by separate injection devices. For example, the engine can injectextra fuel into the exhaust within one or more cylinders prior toexpelling the exhaust. Alternatively, or in addition, reductant can beinjected into the exhaust downstream of the engine.

U.S. Pat. No. 7,082,753 (hereinafter “the '753 patent”) describes anexhaust treatment system with a fuel reformer placed in the exhaust lineupstream from a LNT. The reformer includes both oxidation and reformingcatalysts. The reformer both removes excess oxygen and converts thediesel fuel reductant into more reactive reformate.

The operation of a fuel reformer can be modeled in terms of thefollowing three reactions:0.684CH_(1.85)+O₂→0.684CO₂+0.632H₂O  (1)0.316CH_(1.85)+0.316H₂0→0.316CO+0.608H₂  (2)0.316CO+0.316H₂O→0.316CO₂+0.316H₂  (3)wherein CH_(1.85) represents an exemplary reductant, such as dieselfuel, with a 1.85 ratio between carbon and hydrogen. Reaction (1) isexothermic complete combustion by which oxygen is consumed. Reaction (2)is endothermic steam reforming. Reaction (3) is the water gas shiftreaction, which is comparatively thermal neutral and is not of greatimportance in the present disclosure, as both CO and H₂ are effectivefor regeneration.

The inline reformer of the '753 patent is designed to be rapidly heatedand to then catalyze steam reforming. Temperatures from about 500 toabout 700° C. are said to be required for effective reformate productionby this reformer. These temperatures are substantially higher thantypical diesel exhaust temperatures. The reformer is heated by injectingfuel at a rate that leaves the exhaust lean, whereby Reaction (1) takesplace. After warm up, the fuel injection rate is increased to provide arich exhaust.

Depending on such factors as the exhaust oxygen concentration, the fuelinjection rate, and the exhaust temperature, the inline reformer of the'753 patent tends to either heat or cool as reformate is produced. Intheory, heating can be limited by increasing the fuel injection rate andthereby increasing the rate of endothermic reaction (2). In practice,due to differences in the locations at which reactions (1) and (2) occurand limitations on one more of heat transfer rates, reformer reactionrates, and the efficiency with which an LNT can use reformate, thereformer cannot always be cooled in this manner. As an alternative, the'753 patent suggests pulsing the fuel injection to the reformer duringLNT regenerations. The reformer cools between fuel pulses and therebyremains within an acceptable operating temperature range.

During denitrations, much of the adsorbed NO_(x) is reduced to N₂,although a portion of the adsorbed NO_(x) is released without havingbeen reduced and another portion of the adsorbed NO_(x) is deeplyreduced to ammonia. The NO_(x) release occurs primarily at the beginningof the regeneration. The ammonia production has generally been observedtowards the end of the regeneration.

U.S. Pat. No. 6,732,507 proposes a hybrid system in which a SCR catalystis configured downstream from the LNT in order to utilize the ammoniareleased during denitration. The LNT is provided with more reductantover the course of regeneration than is required to remove theaccumulated NO_(x) in order to facilitate ammonia production. Theammonia is utilized to reduce NO_(x) slipping past the LNT and therebyimproves conversion efficiency over a stand-alone LNT.

U.S. Pat. Pub. No. 2004/0076565 (hereinafter “the '565 publication”)also describes hybrid systems combining LNT and SCR catalysts. In orderto increase ammonia production, it is proposed to reduce the rhodiumloading of the LNT. In order to reduce the NO_(x) release at thebeginning of the regeneration, it is proposed to eliminate oxygenstorage capacity from the LNT.

In addition to accumulating NO_(x), LNTs accumulate SO_(x). SO_(x) isthe combustion product of sulfur present in ordinarily fuel. Even withreduced sulfur fuels, the amount of SO_(x) produced by combustion issignificant. SO_(x) adsorbs more strongly than NO_(x) and necessitates amore stringent, though less frequent, regeneration. Desulfation requireselevated temperatures as well as a reducing atmosphere. In the case of alean-burn gasoline engine, the temperature of the exhaust can generallybe elevated by engine measures. In the case of a diesel engine, however,it is generally necessary to provide additional heat. Typically, thisheat can be provided through the same types of reactions as those usedto remove excess oxygen from the exhaust. Once the LNT is sufficientlyheated, the exhaust is made rich by measures like those used for LNTdenitration. If an inline reformer is used to make the exhaust rich forLNT desulfation, it may be necessary to pulse the fuel injection overthe course of desulfation to prevent the fuel reformer from overheating.

In spite of advances, a long felt need exists for an affordable andreliable exhaust treatment system that is durable, has a manageableoperating cost (including fuel penalty), and is practical for reducingNO_(x) emissions from diesel engines to an extent that meets U.S.Environmental Protection Agency (EPA) regulations effective in 2010 andother such regulations.

SUMMARY

One of the inventor's concepts relates to a power generation system,comprising a diesel engine and a fuel reformer configured to receive theexhaust from the diesel engine. Two or more separate LNT bricks areconfigured in a parallel valveless arrangement so that eachsimultaneously receives a separate portion of the exhaust leaving thefuel reformer. The LNTs are each adapted and configured tosimultaneously store NO_(x) when the exhaust from the fuel reformer islean and to simultaneously reduce stored NO_(x) and regenerate when theexhaust from the fuel reformer contains reformate. This parallelmulti-brick arrangement reduces the effective length to width ratio ofthe LNTs as a group without the packaging difficulties that occur whenequivalently reducing the length to width ratio with a single LNT brick.

A small length to width ratio is particularly useful in this system forreducing axial temperature gradients within the LNTs during desulfation.When fuel injection is pulsed to limit the inline reformer temperature,it has been observed that significant axial temperature gradientsdevelop within the downstream LNTs; their temperatures increase alongthe direction of flow. Desulfation rates are highly sensitive totemperature. Having the temperatures increasing along the direction offlow can substantially prolong desulfation and concomitant thermaldegradation of the LNTs, particularly considering that sulfur depositsprimarily at the fronts of the LNTs, where the LNTs are coolest.Reducing the length to width ratio ameliorates these gradients. MultipleLNT bricks in a parallel valveless arrangement are largely equivalent toa single LNT with a very small length to width ratio, but can bepackaged more easily than the single brick.

Another concept relates to a method of operating a power generationsystem. The method comprises operating a diesel engine to produce anexhaust containing NO_(x) and SO_(x). The exhaust is channeled through aplurality of LNTs, each comprising a separate brick and each receiving aseparate portion of the exhaust flow. The LNTs adsorb and store a firstportion of NO_(x) and a portion of the SO_(x) from the exhaust. Theexhaust from these LNTs is passed through one or more SCR catalysts thatreduce a second portion of NO_(x) in the exhaust by reactions withammonia under lean conditions. The method further comprises generating afirst control signal to denitrate one or more of the LNTs. In responseto the control signal, a rich exhaust is supplied to the one or more ofthe LNTs, whereby adsorbed NO_(x) is reduced producingammonia-containing exhaust. The ammonia containing exhaust is passedthrough one or more of the SCR catalysts, whereby the SCR catalystsadsorb and store ammonia. A second control signal to desulfate one ormore of the LNTs is also eventually generated. In response to the secondcontrol signal, one or more LNTs are regenerated by heating them andmaking the exhaust supplying them rich. The manner of making the exhaustrich is such that the temperatures in the LNTs being desulfated increasein the direction of the exhaust flow. The provision of multiple LNTseach receiving a separate portion of the exhaust flow mitigates thetemperature gradients that develop in the LNTs during desulfation.

The primary purpose of this summary has been to present certain of theinventor's concepts in a simplified form to facilitate understanding ofthe more detailed description that follows. This summary is not acomprehensive description of every one of the inventor's concepts orevery combination of the inventor's concepts that can be considered“invention”. Other concepts of the inventor will be conveyed to one ofordinary skill in the art by the following detailed description togetherwith the drawings. The specifics disclosed herein may be generalized,narrowed, and combined in various ways with the ultimate statement ofwhat the inventor claims as his invention being reserved for the claimsthat follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary power generationsystem.

FIG. 2 is a plot of temperatures and reductant concentration in acomparison power generation over the course of LNT desulfation.

FIG. 3 is a schematic of the power generation system that produced thedata plotted in FIG. (2).

FIG. 4 is a schematic illustration of another exemplary power generationsystem.

FIG. 5 is a schematic illustration of yet another exemplary powergeneration system.

FIG. 6 is a schematic illustration of a further exemplary powergeneration system.

DETAILED DESCRIPTION

FIG. 1 is a schematic of an exemplary power generation system 100embodying one of the inventor's concepts. The power generation system100 comprises an engine 101 and an exhaust aftertreatment system 102.The exhaust aftertreatment system 102 includes a controller 103, a fuelinjector 104, a fuel reformer 105, a plurality of lean NO_(x)-traps(LNT) 106 (including at least two LNTs 106 more specifically identifiedas 106 a and 106 b), and a plurality of ammonia-SCR catalysts 107. Thecontroller 103 may be an engine control unit (ECU) that also controlsthe exhaust aftertreatment system 102 or may include several controlunits that collectively perform these functions.

During lean operation (a lean phase), the LNTs 106 adsorb a firstportion of the NO_(x) from the exhaust. The ammonia-SCR catalysts 107may have ammonia stored from a previous regeneration of the LNTs 106 (arich phase). If the ammonia-SCR catalysts 107 contain stored ammonia,they remove a second portion of the NO_(x) from the lean exhaust.

From time to time, the LNTs 106 must be regenerated in a rich phase toremove accumulated NO_(x) (denitrated). Denitration may involve heatingthe reformer 105 to an operational temperature and then injecting fuelusing the fuel injector 104 to make the exhaust rich. The fuel reformer105 uses the injected fuel to consume most of the oxygen from theexhaust while producing reformate. The reformate thus produced reducesNO_(x) adsorbed in the LNTs 106. Some of this NO_(x) is reduced to NH₃,most of which is captured by the ammonia-SCR catalysts 107 and used toreduce NO_(x) during a subsequent lean phase.

From time to time, the LNTs 106 must also be regenerated to removeaccumulated sulfur compounds (desulfated). Desulfation involves heatingthe reformer 105 to an operational temperature, heating the LNTs 106 toa desulfating temperature, and providing the heated LNTs 106 with a richatmosphere. Desulfating temperatures vary, but are typically in therange from about 500 to about 800° C., with optimal temperaturestypically in the range of about 650 to about 750 ° C. Below a minimumtemperature, desulfation is very slow. Above a maximum temperature, theLNTs 106 may be damaged.

The primary means of heating the LNTs 106 is heat convection from thereformer 105. To generate this heat, fuel can be supplied to thereformer 105 under lean conditions, whereby the fuel combusts in thereformer 105. Once the reformer 105 is heated, the fuel injection ratecan be controlled to maintain the temperature of the reformer 105 whilethe LNTs 106 are heating.

The LNTs 106 can also be heated in part by combustion within them.Heating the LNTs 106 in part in this way reduces the peak temperaturesat which the reformer 105 must be operated. One method of achievingcombustion within the LNTs 106 is to design and operate the fuelreformer 105 in such a way that a portion of the fuel supplied to thefuel reformer 105 slips to the LNTs 106. For example, the catalystloading of the fuel reformer 105 or its mass transfer coefficient can bekept low to facilitate this mechanism. Another method of achievingcombustion in the LNTs 106 is to use rapid cycling between rich and leanphases. Oxygen for the lean phases can mix with fuel or reformate fromthe rich phases to combust in the LNTs 106. This mixing and combustioncan be facilitated by a capacity of the LNTs 106 to adsorb reductants oroxygen.

Even when the LNTs 106 are not specifically designed to adsorb eitherreductants or oxygen, it has become evident that when fuel is pulsed tothe fuel reformer 105 in order to maintain its temperature over thecourse of a desulfation, reductant and oxygen mix and combust in theLNTs 106. Data regarding this phenomenon are provided in FIG. 2.

The data in FIG. 2 were gathered for a power generation system 300configured as illustrated in FIG. 3. In the system 300 of FIG. 3, twoLNT bricks 106 a and 106 b are arranged in series. The LNTs 106 areprovided in two separate bricks in the system 300 to give a target totalLNT volume using conventionally sized LNT bricks. During desulfation,the fuel injection is pulsed to give the reformate concentration profileillustrated by line 201 (CO) and line 202 (H₂) in FIG. 2. Line 203 plotstemperature readings obtained from a thermocouple in the LNT brick 106 a2.5 cm from its entrance. Line 204 plots temperature readings obtainedfrom a thermocouple in the LNT brick 106i a 2.5 cm from its exit. Line205 plots temperature readings obtained from a thermocouple in the LNTbrick 106 b 2.5 cm from its exit. Both LNTs were about 24 cm long and 15cm in diameter. The plots show that peak temperatures increase along thedirection of flow, with peak temperatures near the exit of the two bricksystem being about 150° C. higher than peak temperatures near the frontof the system.

The inventor's concept is to replace a series arrangement of LNTs suchas illustrated by FIG. 3 with a parallel arrangement of LNTs such asillustrated by FIG. 1. By reducing the collective lengths of the LNTs106, the axial temperature gradients can be ameliorated. Temperaturesstill increase along the direction of flow when fuel injection ispulsed, but to a lesser degree. Axial conduction through the substratesof the LNT bricks smoothes the temperature profiles. The area availablefor this transport is increased and the distance over which heat must betransported is reduced when the LNTs 106 are arranged in parallel.

For simplicity of representation, FIG. 1 shows only two separate LNTbricks arranged in parallel. Preferably, however, more than two separateLNT bricks are used in order to achieve a very small overall effectivelength to width ratio for the LNT in comparison to the length to widthratios of the individual LNT bricks. Preferably, three or more LNTsbricks are used. More preferably, four or more separate LNT bricks areused.

Preferably, the equivalent diameter to equivalent length ratio of theLNTs 106 collectively is at least about two, more preferably at leastabout three, and still more preferably at least about four. Equivalentdiameter and equivalent length are calculated on the basis of a singlecylindrical LNT brick having the same total frontal area and totalvolume as the LNTs 106 collectively. The equivalent diameter is obtainedby dividing the total frontal area of the LNTs 106 by pi, taking thesquare root, and multiplying by two. The equivalent length is obtainedby dividing the total volume of the LNTs 106 by the total frontal areaof the LNTs 106.

Each of the LNTs 106 is preferably a separate monolith brick. A monolithis a structure providing an array of parallel passages. A brick is acohesive unit, for example, an extruded structure or a structure formedby rolling one or more stacked sheets of metal into a cylinder. Monolithbricks generally have aspect ratios from about 0.5 to about 2.0, with a1.0 aspect ratio being typical. These dimensions provide structuralstability. Bricks with aspect ratios greater than 2.0 are less strongand are more difficult to manufacture and effectively can. Typicaldiameters and lengths of monolith bricks range from about 15 cm to about36 cm. According to the present concept, shorter bricks are preferable,e.g., bricks from about 7 cm to about 15 cm in length.

Each brick preferably provides a high degree of axial heat conductionper unit of surface area. Combustion that produces heat occurs at a rateproportional to the surface area whether the rate of combustion iskinetically or mass transfer rate controlled. For high porositymonoliths, increasing the wall thickness increases the degree of axialheat conduction. Metal conducts heat better than ceramic. A preferredLNT brick according to the inventor's concept is constructed withrelatively thick metal walls. A thick metal wall is about 100 μm orthicker, preferably about 200 μm or thicker, more preferably about 400μm or thicker.

The benefit of arranging LNTs 106 in parallel can be realized whether ornot the LNTs 106 are desulfated one at a time. In the power generationsystem 100, the LNTs 106 are desulfated simultaneously using a singlereductant source. One advantage of the power generation system 106 isthat it can be constructed and operated without exhaust system valves.Exhaust valves are undesirable because they lack durability andreliability. Mobile dampers are within the scope of valves for thepurpose of this description. The system 106 divides the flow among thevarious branches passively; the division of flow is independent of thecontrol signals that trigger regeneration.

FIG. 4 is a schematic of an exemplary power generation system 400illustrating a second embodiment of the inventor's concept. The mostsignificant difference between this embodiment and that exemplified bythe power generation system 100 is that in the power generation system400 each LNT 106 is provided with an independent mechanism for makingthe exhaust supplying it rich, in this case a separate inline reformer105 for each of the exhaust branches 109. This configuration allows oneor more of the LNTs 106 to be regenerated independently of the others.

A significant advantage of independently regenerating the LNTs 106 isthat rich exhaust from LNTs 106 being regenerated can be combined withlean exhaust from LNTs 106 not being regenerated. Oxygen from the leanexhaust can be used to oxidized residual reductants, slipping NO, andH₂S in the rich exhaust.

NO tends to slip from the LNTs 106 being regenerated, particularly atthe start of a regeneration. Some of this NO may be reduced in the SCRcatalysts 107. Some, however, is not so reduced either because oflimitations on the catalyst efficiency or on the amount of availableammonia. NO is environmentally more harmful than NO₂. Oxidizinguntreated NO to NO₂ improves the overall performance of the exhausttreatment system.

H₂S may slip from the LNTs 106 during desulfation. H₂S has an offensiveodor even in very small concentrations. By oxidizing this H₂S to SO₂,the unpleasant odor can be avoided.

Additional benefits are realized if the SCR catalysts 107 are arrangedafter the point in the exhaust line where the lean and rich flows arecombined. FIG. 5 is a schematic of an exemplary power generation system500 in which the flow is combined while the SCR catalyst 107 stillconsists of multiple separate bricks in a parallel arrangement. Thisembodiment realizes the benefits of a combined flow and an arrangementof SCR catalysts 107 that fits compactly with the arrangement of LNTs106 contemplated by the inventor.

One benefit of combining the flows of separately regenerated LNTs 106prior to supplying the combined flow to SCR catalysts 107 is thatammonia produced by the LNTs 106 during the regenerations is distributedto SCR catalysts 107 more evenly in time. This more even distribution intime increases the efficiency with which the ammonia is used. In thecase of a single LNT 106 followed by a single SCR catalyst 107, theammonia concentration in the SCR catalyst 107 is highest immediatelyfollowing regeneration. Immediately following regeneration, NO_(x) slipfrom the LNT 106 is generally at its lowest. As a result, much of theammonia remains in the SCR catalyst 107 for an extended period prior tobeing used to reduce NO_(x). Over this period, a significant portion ofthe stored ammonia can be lost to decomposition. By staggering theregenerations and spreading out the times over which the LNT bricks 106are regenerated and ammonia is produced, the average time that ammoniamust be stored in the SCR catalysts 107 is significantly reduced, whichresults in increased ammonia utilization.

Another benefit is that the environment of the SCR catalysts 107 can bemaintained continuously lean. SCR catalysts function more effectively inthe presence of oxygen. Maintaining a continuously lean environment inthe SCR catalyst 107 can improve its performance and reduce NO_(x) slipduring regenerations.

In the exemplary power generation systems 100, 400, and 500, the exhaustis made rich using inline reformers 105. The concepts, however, extendto methods of making the exhaust rich that do not include or entirelyrely upon inline reformers. The engine 101 can be used remove excessoxygen from the exhaust: the engine 101 could be operated with astoichiometric or rich fuel-air mixture, if the engine is of such adesign that this is possible. Reformate or another reductant other thandiesel fuel can be injected into the exhaust. Excess oxygen can beremoved by combustion of reductant in a device other than a fuelreformer 105, such as an oxidation or three-way catalyst. In addition,it should be noted that diesel fuel can be injected into the exhaust byan engine fuel injector rather than by an exhaust line fuel injector.

At least one DPF will typically be included in a diesel exhaustaftertreatment system. The DPF can be placed at any suitable location.Examples of suitable locations are upstream from the fuel reformer 105,between the fuel reformer 105 and the LNTs 106, between the LNTs 106 andthe SCR catalysts 107, and downstream from the SCR catalysts 107. Apotential advantage of placing the DPF upstream from the LNTs 106 isthat NO_(x) concentrations are high, facilitating continuousregeneration. A potential advantage of placing the DPF downstream fromthe fuel reformer 105 is that oxidation of NO to NO₂ in the fuelreformer 105 can facilitate DPF regeneration. Also, if placed downstreamfrom the fuel reformer 105, the fuel reformer 105 can be used to heatthe DPF for intermittent regeneration.

If the DPF is placed between the fuel reformer 105 and the LNTs 106, theDPF can provide a thermal mass ameliorating temperature excursion in theLNTs 106 during denitrations. Repeated exposure to high temperatures canreduce the life of the LNTs 106. Between the LNTs 106 and the SCRcatalysts 107, the DPF can have a similar effect: protecting the SCRcatalysts 107 from desulfation temperatures; some SCR catalysts undergodegradation if exposed to desulfation temperatures. Downstream from theSCR catalysts 107 may be a preferred location if the DPF has a catalystthat could oxidize NH₃. The preferred location for the DPF depends onthe type of DPF and other particulars of the various system components.

FIG. 6 provides a schematic illustration of an exemplary powergeneration system 600 comprising an exhaust treatment system 602 inwhich a DPF 108 is configured. Other components of the system 600 arethe same as described for the system 500. The DPF 108 is placeddownstream from the LNTs 106 at a point where the exhaust flow isunified. This configuration allows a continuously lean environment to bemaintained in the DPF 108, provided the LNTs 106 are not all regeneratedsimultaneously. The environment in the SCR catalyst 107 would also becontinuously lean. A lean environment allows the DPF 108 to beregenerated simultaneously with desulfation of one or more of the LNTs106. Heat from the desulfations helps achieve soot combustion.Consumption of oxygen in one or more of the LNTs 106 reduces the riskthe DPF 108 will overheat at internal hot spots.

A DPF can be a wall flow filter or a pass through filter and can useprimarily either depth filtration or cake filtration. Cake filtration isthe primary filter mechanism in a wall flow filter. In a wall flowfilter, the soot-containing exhaust is forced to pass through a porousmedium. Typical pore diameters are from about 0.1 to about 1.0 μm. Sootparticles are most commonly from about 10 to about 50 nm in diameter. Ina fresh wall flow filter, the initial removal is by depth filtration,with soot becoming trapped within the porous structure. Quickly,however, the soot forms a continuous layer on an outer surface of theporous structure. Subsequent filtration is through the filter cake andthe filter cake itself determines the filtration efficiency. As aresult, the filtration efficiency increases over time.

In contrast to a wall flow filter, in a flow through filter the exhaustis channeled through macroscopic passages and the primary mechanism ofsoot trapping is depth filtration. The passages may have rough walls,baffles, and bends designed to increase the tendency of momentum todrive soot particles against or into the walls, but the flow is notforced though micro-pores. The resulting soot removal is considereddepth filtration, although the soot is generally not distributeduniformly with the depth of any structure of the filter. A flow throughfilter can also be made from temperature resistant fibers, such asceramic or metallic fibers, that span the device channels. A flowthrough filter can be larger than a wall flow filter having equivalentthermal mass

Diesel particulate filters must be regenerated from time-to-time toremove accumulated soot. Two general approaches to DPF regeneration arecontinuous and intermittent regeneration. In continuous regeneration, acatalyst is provided upstream from the DPF to convert NO to NO₂. N0 ₂can oxidize soot at typical diesel exhaust temperatures and therebyeffectuate continuous regeneration. Intermittent regeneration involvesheating the DPF to a temperature at which soot combustion isself-sustaining in a lean environment. Typically this is a temperaturefrom about 400 to about 600° C., depending in part on what type ofcatalyst coating has been applied to the DPF to lower the soot ignitiontemperature.

While the engine 9 is preferably a compression ignition diesel engine,the various concepts of the inventor are applicable to power generationsystems with lean-burn gasoline engines or any other type of engine thatproduces an oxygen rich, NO_(x)-containing exhaust. For purposes of thepresent disclosure, NO_(x) consists of NO and NO₂.

The power generation system can have any suitable type of transmission.A transmission can be a conventional transmission such as acounter-shaft type mechanical transmission, but is preferably a CVT. ACVT can provide a much larger selection of operating points than can aconventional transmission and generally also provides a broader range oftorque multipliers. The range of available operating points can be usedto control the exhaust conditions, such as the oxygen flow rate and theexhaust hydrocarbon content. A given power demand can be met by a rangeof torque multiplier-engine speed combinations. A point in this rangethat gives acceptable engine performance while best meeting a controlobjective, such as minimum oxygen flow rate, can be selected. Ingeneral, a CVT prevents or minimizes power interruptions duringshifting.

Examples of CVT systems include hydrostatic transmissions, rollingcontact traction drives, overrunning clutch designs, electrics,multispeed gear boxes with slipping clutches, and V-belt tractiondrives. A CVT may involve power splitting and may also include amulti-step transmission.

A preferred CVT provides a wide range of torque multiplication ratios,reduces the need for shifting in comparison to a conventionaltransmission, and subjects the CVT to only a fraction of the peak torquelevels produced by the engine. These can be achieved using a step-downgear set to reduce the torque passing through the CVT. Torque from theCVT passes through a step-up gear set that restores the torque. The CVTis further protected by splitting the torque from the engine, andrecombining the torque in a planetary gear set. The planetary gear setmixes or combines a direct torque element transmitted from the enginethrough a stepped automatic transmission with a torque element from aCVT, such as a band-type CVT. The combination provides an overall CVT inwhich only a portion of the torque passes through the band-type CVT.

The fuel reformer 105 is a device that converts heavier fuels intolighter compounds without fully combusting the fuel. The fuel reformer105 can be a catalytic reformer or a plasma reformer. Preferably, thefuel reformer 105 is a partial oxidation catalytic reformer comprising asteam reforming catalyst. Examples of reformer catalysts includeprecious metals, such as Pt, Pd, and Rh, and oxides of Al, Mg, and Ni,the latter group being typically combined with one or more of CaO, K₂O,and a rare earth metal such as Ce to increase activity. The fuelreformer 105 is preferably small compared to an oxidation catalyst thatis designed to perform its primary functions at temperatures below 450°C. The reformer 105 is generally operative at temperatures within therange of about 450to about 1100° C.

The LNTs 106 can comprise any suitable NO_(x)-adsorbing material.Examples of NO_(x) adsorbing materials include oxides, carbonates, andhydroxides of alkaline earth metals such as Mg, Ca, Sr, and Ba or alkalimetals such as K or Cs. Further examples of NO_(x)-adsorbing materialsinclude molecular sieves, such as zeolites, alumina, silica, andactivated carbon. Still further examples include metal phosphates, suchas phosphates of titanium and zirconium. Generally, the NO_(x)-absorbingmaterial is an alkaline earth oxide. The adsorbent is typically combinedwith a binder and either formed into a self-supporting structure orapplied as a coating over an inert substrate.

The LNTs 106 also comprise a catalyst for the reduction of NO_(x) in areducing environment. The catalyst can be, for example, one or moretransition metals, such as Au, Ag, and Cu, group VIII metals, such asPt, Rh, Pd, Ru, Ni, and Co, Cr, or Mo. A typical catalyst includes Ptand Rh. Precious metal catalysts also facilitate the adsorbent functionof alkaline earth oxide adsorbers.

Adsorbents and catalysts according to the present invention aregenerally adapted for use in vehicle exhaust systems. Vehicle exhaustsystems create restriction on weight, dimensions, and durability. Forexample, a NO_(x) adsorbent bed for a vehicle exhaust system must bereasonably resistant to degradation under the vibrations encounteredduring vehicle operation.

The ammonia-SCR catalysts 107 are catalysts functional to catalyzereactions between NO_(x) and NH₃ to reduce NO_(x) to N₂ in lean exhaust.Examples of SCR catalysts include oxides of metals such as Cu, Zn, V,Cr, Al, Ti, Mn, Co, Fe, Ni, Pd, Pt, Rh, Mo, W, and Ce, zeolites, such asZSM-5 or ZSM-11, substituted with metal ions such as cations of Cu, Co,Ag, Zn, or Pt, and activated carbon. Preferably, the ammonia-SCRcatalysts 107 are designed to tolerate temperatures required todesulfate the LNTs 106.

Although not illustrated in any of the figures, a clean-up catalyst canbe placed downstream from the other aftertreatment device. A clean-upcatalyst is preferably functional to oxidize unburned hydrocarbons fromthe engine 101, unused reductants, and any H₂S released from the LNTs106 and not oxidized by the ammonia-SCR catalyst 107. Any suitableoxidation catalyst can be used. To allow the clean-up catalyst tofunction under rich conditions, the catalyst may include anoxygen-storing component, such as ceria. Removal of H₂S, when required,may be facilitated by one or more additional components such as NiO,Fe₂O₃, MnO₂, CoO, and CrO₂.

The invention as delineated by the following claims has been shownand/or described in terms of certain concepts, components, and features.While a particular component or feature may have been disclosed hereinwith respect to only one of several concepts or examples or in bothbroad and narrow terms, the components or features in their broad ornarrow conceptions may be combined with one or more other components orfeatures in their broad or narrow conceptions wherein such a combinationwould be recognized as logical by one of ordinary skill in the art.Also, this one specification may describe more than one invention andthe following claims do not necessarily encompass every concept, aspect,embodiment, or example described herein.

1. A power generation system, comprising: a diesel engine operative toproduce an exhaust containing NO_(x) a fuel reformer configured toreceive the exhaust and operative to produce reformate when the fuelreformer is sufficiently warm, the exhaust is rich, and the exhaustcontains diesel fuel; two or more separate LNT bricks configured in aparallel valveless; arrangement wherein each simultaneously receives aseparate portion of the exhaust leaving the fuel reformer, the LNTs eachbeing adapted and configured to simultaneously store NO_(x) when theexhaust from the fuel reformer is lean and to simultaneously reducestored NO_(x) and regenerate when the exhaust from the fuel reformer isrich and contains reformate.
 2. The power generation system of claim 1,wherein the two or more separate LNT bricks comprise at least threeseparate LNT bricks.
 3. The power generation system of claim 1, whereineach LNT brick is a monolith from about 7 cm to about 15 cm in length.4. The power generation system of claim 1, wherein: an equivalentdiameter to equivalent length ratio of the two or more separate LNTbricks is at least about two; the equivalent diameter is obtained bydividing the total frontal area of the two or more separate LNT bricksby pi, taking the square root, and multiplying by two; and theequivalent length is obtained by dividing the total volume of the two ormore separate LNT bricks by the total frontal area of the two or moreseparate LNT bricks.
 5. The power generation system of claim 4, whereinthe equivalent diameter to equivalent length ratio of the two or moreseparate LNT bricks is at least about three.
 6. The power generationsystem of claim 4, wherein the equivalent diameter to equivalent lengthratio of the two or more separate LNT bricks is at least about four. 7.A method of operating a power generation system, comprising: operating adiesel engine to produce an exhaust containing NO_(x) and SO_(x);channeling the exhaust through a plurality of LNTs that adsorb and storea first portion of NO_(x) and a portion of the SO_(x) from the exhaust;passing the exhaust from the plurality of LNTs through one or more SCRcatalysts that reduce a second portion of NO_(x) in the exhaust byreaction with ammonia under lean conditions; generating a first controlsignal to denitrate a first one or more of the LNTs; in response to thecontrol signal, supplying rich exhaust to the first one or more of theLNTs, whereby adsorbed NO_(x) in the first one or more LNTs is reducedproducing ammonia-containing exhaust; passing the ammonia containingexhaust through one or more of the SCR catalysts, whereby the one ormore ammonia-SCR catalysts adsorb and store ammonia; generating a secondcontrol signal to desulfate one or more of the LNTs; and in response tothe second control signal, desulfating a second one or more LNTs byheating the second one or more LNTs and making the exhaust supplying thesecond one or more LNTs rich such that over the course of thedesulfation, the temperatures in the second one or more LNTs increase inthe direction of the exhaust flow; wherein the LNTs each comprise aseparate brick and each LNT simultaneously receives a separate portionof the exhaust.
 8. The method of claim 7, wherein the exhaust is dividedamong the plurality of LNTs by static structures that do not move inresponse to either control signal.
 9. The method of claim 7, wherein thetemperatures of the second one or more LNTs increase in the direction offlow during desulfation due to reactions involving residual oxygencarried by the exhaust during desulfation.
 10. The method of claim 7,wherein the temperatures of the second one or more LNTs increases in thedirection of flow during desulfation by reactions between reductants andoxygen stored in the LNTs.
 11. The method of claim 7, wherein supplyingrich exhaust to the first one or more of the LNTs comprises injectinghydrocarbons into the exhaust and passing the exhaust through a fuelreformer.
 12. The method of claim 11, further comprising heating thefuel reformer in response to the first control signal in preparation forsupplying rich exhaust to the first one or more of the LNTs and whereinthe fuel reformer comprises an effective amount of a steam reformingcatalyst.
 13. The method of claim 7, wherein the first one or more LNTsand the second one or more LNTs each comprise all the LNTs.
 14. Themethod of claim 13, wherein a single fuel reformer is configured tosupply rich exhaust to all the LNTs.
 15. The method of claim 14, whereinthe fuel reformer is configured to receive all the exhaust from thediesel engine and the fuel reformer produces reformate by steamreforming reactions.
 16. The method of claim 7, wherein there are threeor more LNTs each comprising a monolith brick from about 7 cm to about15 cm in length.
 17. The method of claim 7, wherein the plurality ofLNTs collectively have an equivalent diameter to equivalent length ratioof at least about three; the equivalent diameter is obtained by dividingthe total frontal area of the two or more separate LNT bricks by pi,taking the square root, and multiplying by two; and the equivalentlength is obtained by dividing the total volume of the two or moreseparate LNT bricks by the total frontal area of the two or moreseparate LNT bricks.
 18. The method of claim 7, wherein the equivalentdiameter to equivalent length ratio of the two or more separate LNTbricks is at least about four.